Howe do i fatten up a extremely thin leopard geko ! and fast ?

my leopard geko is female , we got her from a friend who no longer wanted her as a new lizzard, weve had her for around a year now , and she was doing fine , all of a sudden she has become really thin and will not eat , ive tried to force feed her and she eats from me , but soon as i stop , and try to let her do it alone she becomes super skinny once more , i also have a large male , he likes to lay on her and im scared he will crush her , should i remove her and separate them till she is well again , and will i be able to reunite them if she recovers , pleaseee hellpp ? x